| Abstract
| - The degree of sp-hybridization, that is, stereochemical activity, of the Pb(II) lone pair is tuned in three lead(II)/Au(CN)2− coordination polymers by incorporating chelating N-donor ligands of differing basicities, thereby influencing the supramolecular structures and properties. X-ray crystallography, solid-state 207Pb MAS NMR, luminescence measurements, and relativistic theoretical calculations each clearly provide valuable insight into the nature of the stereochemical activity of the lone pair in these lead(II) coordination polymers.
- The reaction of Pb(ClO4)2·xH2O, an ancillary ligand L, and two equivalents of Au(CN)2− gave a series of crystalline coordination polymers, which were structurally characterized. The ligands were chosen to represent a range of increasing basicity, to influence the stereochemical activity (i.e., p-orbital character) of the Pb(II) lone pair. The Pb(II) center in [Pb(1,10-phenanthroline)2][Au(CN)2]2 (1) is 8-coordinate, with a stereochemically inactive lone pair; all 8 Pb−N bonds are similar. The Au(CN)2− units propagate a 2-D brick-wall structure. In [Pb(2,2′-bipyridine)2][Au(CN)2]2 (2), the 8-coordinate Pb(II) center has asymmetric Pb−N bond lengths, indicating moderate lone pair stereochemical activity; the supramolecular structure forms a 1-D chain/ribbon motif. For [Pb(ethylenediamine)][Au(CN)2]2 (3), the Pb(II) is only 5-coordinate and extremely asymmetric, with Pb−N bond lengths from 2.123(7) to 3.035(9) Å; a rare Pb−Au contact of 3.5494(5) Å is also observed. The Au(CN)2− units connect the Pb(ethylenediamine) centers to form 1-D zigzag chains which stack via Au−Au interactions of 3.3221(5) Å to yield a 2-D sheet. 207Pb MAS NMR of the polymers indicates an increase in both the chemical shielding span and isotropic chemical shift with increasing Pb(II) coordination sphere anisotropy (from δiso = −2970 and Ω = 740 for 1 to δiso = −448 and Ω = 3980 for 3). The shielding anisotropy is positively correlated with Pb(II) p-character, and reflects a direct connection between the NMR parameters and lone-pair activity. Solid-state variable-temperature luminescence measurements indicate that the emission bands at 520 and 494 nm, for 1 and 2, respectively, can be attributed to Pb → L transitions, by comparison with simple [Pb(L)2](ClO4)2 salts. In contrast, two emission bands for 3 at 408 and 440 nm are assignable to Au−Au and Pb−Au-based transitions, respectively, as supported by single-point density-functional theory calculations on models of 3.
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